Method for fabricating a semiconductor device that includes light beam irradiation to separate a semiconductor layer from a single crystal substrate

ABSTRACT

A spacer layer is formed on a single-crystal substrate and an epitaxially grown layer composed of a group III-V compound semiconductor layer containing a nitride or the like is further formed on the spacer layer. The epitaxially grown layer is adhered to a recipient substrate. The back surface of the single-crystal substrate is irradiated with a light beam such as a laser beam or a bright line spectrum from a mercury vapor lamp such that the epitaxially grown layer and the single-crystal substrate are separated from each other. Since the forbidden band of the spacer layer is smaller than that of the single-crystal substrate, it is possible to separate the thin semiconductor layer from the substrate by decomposing or fusing the spacer layer, while suppressing the occurrence of a crystal defect or a crack in the epitaxially grown layer.

BACKGROUND OF THE INVENTION

The present invention relates to a method for fabricating asemiconductor device for use as, e.g., a short-wavelength light-emittingdiode, a short-wavelength semiconductor laser, or a high-temperature andhigh-speed transistor.

A nitride semiconductor which has a large optical band gap (e.g., GaNhas an optical band gap of about 3.4 eV at a room temperature) has beenconventionally used as a material for implementing a visiblelight-emitting diode which emits light in a relatively short wavelengthregion such as green, blue, or white light or a short-wavelengthsemiconductor laser which would be used for future high density opticaldisks. In particular, a nitride semiconductor has been used prevalentlyfor the active layer of a light-emitting diode. As a light source for aread/write operation to a high-density optical disk, thecommercialization of a blue or blue-purple laser has been in strongdemand.

It is general practice to form each of nitride semiconductor layerscomposing a device on a sapphire substrate having a principal surfacesubstantially coincident with the (0001) plane by typical metal organicchemical vapor deposition. In the case of fabricating a semiconductorlaser, it is necessary to form cleavage planes serving as mirrors of thecavity at the edge portions of the semiconductor laser structure of thenitride semiconductor layers with a waveguide structure and electrodes.However, it has been difficult to cleave the entire substrate since thecrystal structure of the sapphire substrate has rotated by 30° from thatof the nitride semiconductor in the c plane ((0001) surface)) andsapphire is hard to be cleaved. This has prevented the formation ofsatisfactory resonator surfaces (mirrors) and made it difficult toachieve a high performance semiconductor laser, especially with a lowthreshold current.

To solve the problem, there has been proposed a method of epitaxiallygrowing nitride semiconductor layers, adhering the nitride semiconductorlayers to a recipient substrate made of a material that allowssuccessful formation of cleavage planes of the nitride semiconductorlayers thereon, separating the nitride semiconductor layers and thesapphire substrate from each other, and thereby cleaving the nitridesemiconductor layers or the recipient substrate. In accordance with themethod, the separation of the nitride semiconductor layers and thesapphire substrate is accomplished by irradiation of a laser beam fromthe back surface of the sapphire substrate and thereby decomposing orfusing a GaN layer and the like present at the interface with thesapphire substrate. The method uses, e.g., an Si (001) substrate as arecipient substrate. By bringing the cleavage plane of Si and thecleavage plane of GaN into parallel relation upon adhesion, the nitridesemiconductor layers can be formed with two flat resonator surfacesparallel to each other. This allows such a high performancesemiconductor laser with a lower threshold current and a longerlifetime.

A description will be given to a method for fabricating theaforementioned nitride semiconductor device. FIGS. 16A to 16D arecross-sectional view illustrating the conventional method forfabricating a nitride semiconductor device.

First, in the step shown in FIG. 16A, an epitaxially grown layer 103having a multilayer structure including a GaN layer, an AlGaN layer, andan InGaN layer and having a pn junction portion is formed by, e.g.,metal organic chemical vapor deposition (MOCVD) on a sapphire substrate101 (wafer). In the case of fabricating a semiconductor laser, awaveguide structure has been incorporated into the epitaxially grownlayer 103 by using a regrowth technique, selective etching of theepitaxial layer, or the like.

Next, in the step shown in FIG. 16B the epitaxially grown layer 103 isadhered to an Si substrate 104 having a principal surface substantiallycoincident with the (001) plane. In the step shown in FIG. 16C, the backsurface of the sapphire substrate 101 is irradiated with a KrF excimerlaser beam (at a wavelength of 248 nm).

FIG. 17 is an energy band diagram showing band states in the sapphiresubstrate 101 and in the GaN layer included in the nitride semiconductorlayers. Since the band gap (optical band gap) of the sapphire substrate101 is large, as shown in the drawing, the output of the KrF excimerlaser beam is not absorbed by the sapphire substrate 101. Because theband gap (optical band gap) of the GaN layer is small, the laser beamused for irradiation is absorbed by the GaN layer so that, if the powerof the laser is extremely high, the energy of the laser beam is consumedto break chemical bonds so that the bonds in the GaN layer are broken inthe vicinity of the interface with the sapphire substrate 101.

Consequently, the sapphire substrate 101 and the epitaxially grown layer103 are separated from each other, as shown in FIG. 16D. Thereafter, theprocess of forming an electrode which comes in contact with theepitaxially grown layer 103 on the Si substrate 104, cleaving thesubstrate (in the case of fabricating the semiconductor laser), and thelike is performed. In the case of fabricating the semiconductor laser,the epitaxially grown layer 103 and the Si substrate 104 are adhered toeach other such that the <11-20> direction of the GaN layer and the<110> direction of the Si substrate are in parallel relation for easycleavage.

The foregoing fabrication method allows formation of flat resonatorsurfaces of the semiconductor laser. Since the nitride semiconductorlayers are adhered to the Si substrate with superior heat dissipation,the semiconductor laser is expected to have a longer lifetime.

However, the foregoing method for fabricating a nitride semiconductordevice has the following problems.

In the step shown in FIG. 16C, the irradiation with the KrF excimerlaser beam increases the probability of a crystal defect or a crackoccurring in the region of the GaN layer adjacent the interface betweenthe epitaxially grown layer 103 and the sapphire substrate 101. Thisnarrows down an optimum range in which the power of the KrF excimerlaser. If the epitaxially grown layer is as thin as about 4 μm, acrystal defect and a crack extend the surface of the epitaxially grownlayer so that it is necessary to form the nitride semiconductor layers(epitaxially grown layer) having a total film thickness as large asabout, e.g., 10 μm. If the nitride semiconductor layers on the sapphiresubstrate are increased in film thickness, however, the bowing of theentire wafer during cooling after epitaxial growth becomes conspicuousdue to the different thermal expansion coefficients of the sapphiresubstrate and the nitride semiconductor layers so that it is difficultto adhere the flat recipient substrate to the wafer.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor fabricating a semiconductor device having a very thin epitaxiallygrown laser separated from the substrate for the epitaxial growthcontaining a reduced number of crystal defects.

A first method for fabricating a semiconductor device according to thepresent invention is a method for fabricating a semiconductor devicehaving a semiconductor layer formed by epitaxial growth from asingle-crystal substrate, the method comprising the steps of: (a)forming a spacer layer having an optical band gap smaller than anoptical band gap of a lowermost portion of the semiconductor layer suchthat an upper surface of the single-crystal substrate is covered withthe spacer layer; (b) forming the semiconductor layer on the spacerlayer; and (c) irradiating the spacer layer with a light beam having anenergy smaller than an optical band gap of the single-crystal substrateand larger than the optical band gap of the spacer layer through a backsurface of the single-crystal substrate to separate the semiconductorlayer from the single-crystal substrate.

Since the method irradiates the spacer layer with the light beam in thestep (c) with the spacer layer smaller in optical band gap width thanthe single-crystal substrate being interposed between the single-crystalsubstrate and the semiconductor layer, the energy of the light beam isprimarily absorbed by the spacer layer. As a result, the portion of thespacer layer adjacent the interface with the single-crystal substrate ismainly decomposed or fused so that the single-crystal substrate and thesemiconductor layer are separated from each other. This allows theseparation of the semiconductor layer as thin as, e.g., 5 μm or lessfrom the substrate at a low light power density without causing acrystal defect or a crack in the semiconductor layer.

The step (b) can include forming a compound semiconductor layercontaining nitrogen as the semiconductor layer.

The step (b) can include forming a group III-V compound semiconductorlayer as the semiconductor layer.

The step (a) includes forming a ZnO layer as the spacer layer and thestep (b) includes composing the lowermost portion of the semiconductorlayer of a group III-V compound material containing nitrogen and havingan optical band gap larger than an optical band gap of the ZnO layer.The arrangement allows easy separation of the semiconductor layer andthe single-crystal substrate from each other by using the relativelysmall optical band gap of the ZnO layer. Since the light beam used forthe irradiation is absorbed primarily by the ZnO layer, thesemiconductor layer as thin as, e.g., 5 μm or less can be separated fromthe substrate at a low light power density without causing a crystaldefect or a crack in the semiconductor layer.

The step (a) includes forming, as the spacer layer, a group III-Vcompound semiconductor layer containing nitrogen and the step (b)includes composing the lowermost portion of the semiconductor layer of agroup III-V compound semiconductor layer containing nitrogen and havingan optical band gap larger than the optical band gap of the spacerlayer. The arrangement allows easy separation of the semiconductor layerand the single-crystal substrate from each other by using the relativelysmall optical band gap of the spacer layer. Since the light beam usedfor the irradiation is absorbed primarily by the spacer layer, thesemiconductor layer as thin as, e.g., 5 μm or less can be separated fromthe substrate at a low light power density without causing a crystaldefect or a crack in the semiconductor layer.

The step (a) can include forming an In_(x)Ga_(1-x)N layer (0<x≦1) as thespacer layer and the step (b) can include composing the lowermostportion of the semiconductor layer of an Al_(y)Ga_(1-y)N layer (0<y≦1).

The step (a) includes forming a GaN layer as the spacer layer and thestep (b) includes composing the lowermost portion of the semiconductorlayer of an Al_(y)Ga_(1-y)N layer (0<y≦1). This achieves a reduction inthe thickness of the semiconductor layer.

Preferably, the step (a) includes forming a GaN layer as the spacerlayer and the step (b) includes adjusting a thickness of thesemiconductor layer to a range not less than 0.5 μm and less than 4 μm.

The first method further comprises, after the step (a) and prior to thestep (b), the step of: forming, on the spacer layer, a multilayerportion composed of a plurality of thin films stacked in layers to havegradually varying compositions, wherein the step (b) includes formingthe semiconductor layer on the multilayer portion. Even if a crystaldefect or a crack is caused by the light beam used for the irradiation,the arrangement prevents the crystal defect or crack in the multilayerportion from extending from the multilayer portion to the semiconductorlayer so that the semiconductor layer has an excellent crystallineproperty.

The multilayer portion is a multiple quantum well layer composed ofalternately stacked quantum well layers and barrier layers. Thearrangement allows the formation of a high-performance device using themultiple quantum well layer.

The step (b) includes the substeps of: (b1) forming a plurality ofcovering portions covering the spacer layer in mutually spaced apartrelation; and (b2) forming the semiconductor layer such that the spacerlayer and the plurality of covering portions are covered with thesemiconductor layer. The arrangement allows the formation of asemiconductor layer with an excellent crystalline property by using asmaller number of crystal defects and cracks contained in the portion ofthe semiconductor layer growing laterally along the upper surfaces ofthe covering portions.

In that case, the substep (b2) can include the steps of prior to thesubstep (b1), forming, in a part of the semiconductor layer covering thespacer layer, the plurality of covering portions in mutually spacedapart relation; and after the substep (b1), forming a remaining part ofthe semiconductor layer through a space between the covering portions.

The substep (b2) includes forming the covering portions composed of amultilayer insulating film or a metal film. In addition to achieving theforegoing effects, the arrangement allows the light beam to focus on theinterface between the spacer layer and the single-crystal substrate sothat the occurrence of a single crystal or a crack in the semiconductorlayer is suppressed.

The substep (b2) includes forming the covering portions composed of amaterial lower in thermal conductivity than the spacer layer. Inaddition to achieving the foregoing effects, the arrangement allows theheat to focus on the interface between the spacer layer and thesingle-crystal substrate so that the occurrence of a single crystal or acrack in the semiconductor layer is suppressed.

The step (c) includes performing the irradiation with the light beamhaving an energy smaller than the optical band gap of the lowermostportion of the semiconductor layer. The arrangement allows theseparation of the semiconductor layer and the single-crystal substratethrough the decomposition or fusion of the spacer layer, while morepositively circumventing the decomposition or fusion of thesemiconductor layer.

The first method further comprises, prior to the step (a), the step of:forming, on the single-crystal substrate, a buffer layer having anoptical band gap larger than the energy of the light beam used for theirradiation in the step (c) such that the buffer layer reducesdistortion resulting from a lattice mismatch between the spacer layerand the single-crystal substrate in the step (c), wherein the step (a)includes forming the spacer layer on the buffer layer. The arrangementmost positively prevents the occurrence of a defect in the spacer layer.As a result, a semiconductor layer with an excellent crystallineproperty is obtained.

The step of forming the buffer layer includes forming, as the bufferlayer, an AlN buffer layer having a thickness in the range of 0.5 μm to2 μm. The arrangement allows the formation of a semiconductor layer witha remarkably excellent crystalline property.

If the first method further comprises, prior to the step (a), the stepof: forming, on the single-crystal substrate, an AlN buffer layer havinga thickness of 0.5 μm or more, the step (a) preferably includes formingan In_(x)Ga_(1-x)N layer (0<x≦1) or a GaN layer as the spacer layer andthe step (b) preferably includes forming the semiconductor layer on thespacer layer such that the lowermost portion of the semiconductor layeris composed of an Al_(y)Ga_(1-y)N layer (0<y≦1).

The step (c) includes performing the irradiation with the light beamfrom a laser oscillating pulsatively. The arrangement increases theoutput power of the light beam, which rapidly decomposes or fuses thespacer layer and allows the semiconductor layer to be separated from thesubstrate.

The step (c) includes irradiating the back surface of the single-crystalsubstrate with a bright line spectrum from a mercury vapor lamp. Thearrangement allows, e.g., a group III-V compound semiconductor layercontaining nitrogen and having an optical band gap larger than theenergy of a bright spectrum line at 365 nm and a group III-V compoundsemiconductor layer containing nitrogen and having an optical band gapsmaller than the energy of the bright spectrum line at 365 nm to beformed by varying the compositions of films. As a result, it becomespossible to form the spacer layer and the semiconductor layer from thegroup III-V compound semiconductor layers containing nitrogen.

The step (c) includes heating the single-crystal substrate. Thearrangement reduces a stress caused in a film by the different thermalexpansion coefficients during cooling down after the formation of thespacer layer and facilitates the separation of the semiconductor layerformed on the single-crystal substrate having a large area therefrom.

Preferably, a temperature to which the single-crystal substrate isheated in the step (c) is in the range of 400° C. to 750° C.

The step (c) includes performing the irradiation with the light beamsuch that an optical flux from a light source scans the entire surfaceof the single-crystal substrate. The arrangement facilitates theseparation of the semiconductor layer formed on the single-crystalsubstrate having a large area therefrom.

Preferably, a substrate selected from a sapphire substrate, an SiCsubstrate, an MgO substrate, an LiGaO₂ substrate, an LiGa_(x)Al_(1-x)O₂(0≦x≦1) mixed crystal substrate, and an LiAlO₂ substrate is used as thesingle-crystal substrate. The use of the sapphire substrate improves theinitial growth of a group III-V compound and allows a group III-Vsemiconductor layer containing nitrogen and having an excellentcrystalline property to be formed thereon. The use of the SiC substrate,the MgO substrate, the LiGaO₂ substrate, or the LiGa_(x)Al_(1-x)O₂ mixedcrystal substrate brings the lattice constant of the single-crystalsubstrate closer to that of the group III-V compound semiconductor layerand allows the semiconductor layer composed of a group III-V compound,containing nitrogen, and having an excellent crystalline property to beformed thereon.

The first method may further comprise, after the step (b) and prior tothe step (c), the step of fixing, onto the semiconductor layer, arecipient substrate composed of a material different from a materialcomposing the semiconductor layer, wherein the step (c) includestransferring the semiconductor layer from the single-crystal substrateto the recipient substrate. Alternatively, the first method can furthercomprise, after the step (c), the step of: fixing, onto thesemiconductor layer, a recipient substrate composed of a materialdifferent from a material composing the semiconductor layer andtransferring the semiconductor layer from the single-crystal substrateto the recipient substrate. In either case, the arrangement allows theseparation of the semiconductor layer that has been fixed to therecipient substrate from the single-crystal substrate. Accordingly, itbecomes possible to adjust the respective crystal orientations of therecipient substrate and the semiconductor layer such that the respectivecleavage planes of the recipient substrate and the semiconductor layerare positioned in a common plane. Even if the cleavage plane of thesingle-crystal substrate is not coincident with that of thesemiconductor layer grown epitaxially thereon or if the single-crystalsubstrate is composed of a material difficult to cleave, the edgeportions thereof can be formed with flat cleavage planes by selectivelyusing a material which can be cleaved simultaneously with thesemiconductor layer to compose the recipient substrate. If thesemiconductor device is, e.g., a semiconductor laser, therefore, ahigh-light-output semiconductor laser using the flat cleavage planes asthe mirror surfaces of the cavity is obtained.

A substrate selected from an Si substrate, a GaAs substrate, a GaPsubstrate, and an InP substrate is used as the recipient substrate. Thisallows easy formation of excellent cleavage planes.

A second method for fabricating a semiconductor device according to thepresent invention comprises the steps of: (a) forming an AlN bufferlayer having a thickness of 0.5 μm or more on an single-crystalsubstrate; (b) forming a semiconductor layer covering the AlN bufferlayer and having a lowermost portion composed of an Al_(y)Ga_(1-y)Nlayer (0≦y≦1); and (c) irradiating the semiconductor layer with a lightbeam having an energy smaller than an optical band gap of the AlN bufferlayer and larger than an optical band gap of the lowermost portion ofthe semiconductor layer through a back surface of the single-crystallayer to separate the semiconductor layer from the single-crystalsubstrate.

The method allows the formation of a semiconductor layer with aremarkably excellent crystalline property on the AlN buffer layer.

A third method for fabricating a semiconductor device according to thepresent invention is a method for fabricating a semiconductor devicehaving a semiconductor layer formed by epitaxial growth from asingle-crystal substrate, the method comprising the steps of: (a)forming a multilayer portion for preventing extension of a defect suchthat an upper surface of the single-crystal substrate is coveredtherewith, the multilayer portion being composed of a plurality of thinfilms stacked in layers to have gradually varying compositions; and (b)forming the semiconductor layer on the multilayer portion.

In accordance with the method, even if an initially grown layer incontact with a bulk single-crystal substrate or with a single-crystalsubstrate contains a large number of crystal defects and cracks, thecrystal defects and cracks are prevented from extending from thesingle-crystal substrate to the semiconductor layer so that asemiconductor layer with an excellent crystalline property is obtained.

The step (a) includes forming the multilayer portion by alternatelystacking in layers two thin films having different compositions. Thearrangement more positively achieves the effect of preventing thecrystal defects and cracks contained in the single-crystal substratefrom extending therefrom to the semiconductor layer.

The step (a) includes forming the multilayer portion having a multiplequantum well structure in which the plurality of thin films are quantumwell layers and barrier layers. The arrangement allows the formation ofa high-performance semiconductor device using a multiple quantum wellstructure.

The step (a) includes doping either the quantum well layers or thebarrier layers with a dopant at such a high concentration as to allowcarriers to spread out upon application of an ON voltage. Thearrangement provides a low-resistance semiconductor layer free fromcracks and a high-performance semiconductor device. If the semiconductorlayer is transferred to the recipient substrate after the step (b) to beprovided on the active region of a laser light-emitting device, theresistance of the multilayer portion is low relative to a supply ofcarriers to the active region so that the light output of thesemiconductor laser is increased.

A semiconductor device according to the present invention comprises: asubstrate; a semiconductor layer provided on the substrate and includingan active layer serving as a light-emitting region; and a multiplequantum well layer provided on the semiconductor layer and composed ofquantum well layers and barrier layers which are alternately stacked.

In the arrangement, the multiple quantum well layer is provided on thesemiconductor layer serving as the active region of the laserlight-emitting device so that a high-performance laser device using themultiple quantum well layer is obtained.

Either the quantum well layers or the barrier layers contain a dopant atsuch a high concentration as to allow carriers to spread out uponapplication of an ON voltage. In the arrangement, the light output ofthe semiconductor laser can be increased by using the resistance of themultiple quantum well layer which is low relative to the supply ofcarriers to the active region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are cross-sectional views illustrating a method forfabricating a semiconductor device according to a first embodiment ofthe present invention;

FIG. 2 is a cross-sectional view showing an example of a multilayerstructure formed in the step shown in FIG. 1A;

FIG. 3 is a view showing an example of a multilayer structure formed inthe step shown in FIG. 1D;

FIG. 4 is an energy band diagram showing respective band structures in asapphire substrate, a ZnO layer, and a GaN layer included in anepitaxially grown layer according to the first embodiment;

FIGS. 5A to 5D are cross-sectional views illustrating a method forfabricating a semiconductor device according to a second embodiment ofthe present invention;

FIG. 6 is an energy band diagram showing respective band structures in asapphire substrate, an In_(0.1)Ga_(0.9)N layer, and a GaN layer includedin an epitaxially grown layer according to the second embodiment;

FIGS. 7A to 7D are cross-sectional views illustrating a method forfabricating a semiconductor device according to a third embodiment ofthe present invention;

FIG. 8 is an energy band diagram showing respective band structures in asapphire substrate, an In_(0.1)Ga_(0.9)N layer, and an Al_(0.1)Ga_(0.9)Nlayer included in an epitaxially grown layer according to the thirdembodiment;

FIGS. 9A to 9D are cross-sectional views illustrating a method forfabricating a semiconductor device according to a fourth embodiment ofthe present invention;

FIG. 10 is an energy band diagram showing respective band structures ina sapphire substrate, an AlN buffer layer, an In_(0.1)Ga_(0.9)N layer,and a GaN layer included in an epitaxially grown layer according to thefourth embodiment;

FIGS. 11A to 11D are cross-sectional views illustrating a method forfabricating a semiconductor device according to a fifth embodiment ofthe present invention;

FIG. 12 is an energy band diagram showing respective band structures ina sapphire substrate, a ZnO layer, an AlGaN multilayer portion, and aGaN layer in an epitaxially grown layer according to the fifthembodiment;

FIG. 13 is a cross-sectional view showing a variation of the fifthembodiment in which an AlGaN multilayer portion is formed to have amultiple quantum well structure;

FIGS. 14A to 14D are cross-sectional views illustrating a method forfabricating a semiconductor device according to a sixth embodiment ofthe present invention;

FIG. 15 is a cross-sectional view showing an exemplary structure of anSiO₂/TiO₂ multilayer film according to the sixth embodiment;

FIGS. 16A to 16D illustrate a conventional method for fabricating asemiconductor device;

FIG. 17 is a diagram showing a diaphragm in the optical band gap of eachof layers in the conventional semiconductor device;

FIG. 18 is an energy band diagram showing respective band structures ina sapphire substrate, an epitaxially grown layer, and the like accordingto a variation of the third embodiment;

FIG. 19 is an energy band diagram showing respective band structures ina sapphire substrate, an epitaxially grown layer, and the like accordingto a variation of the fourth embodiment; and

FIG. 20 is an energy band diagram showing respective band structures ina sapphire substrate, an epitaxially grown layer, and the like accordingto a variation of a seventh embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Embodiment 1

FIGS. 1A to 1D are cross-sectional views illustrating a method forfabricating a semiconductor device according to a first embodiment ofthe present invention.

First, in the step shown in FIG. 1A, a sapphire substrate 1 (wafer)having a principal surface substantially coincident with the (0001)plane (c plane) is prepared. Then, a ZnO layer 2 having a thickness ofabout 100 nm and serving as a spacer layer is formed by, e.g., RFsputtering on the sapphire substrate 1. Subsequently, an epitaxiallygrown layer 3 (with a thickness of 5 μm) having a multilayer structureincluding a GaN layer, an AlGaN layer, and an InGaN layer, each of whichis of p-type, of n-type, or undoped, is formed on the ZnO layer 2 bymetal organic chemical vapor deposition (MOCVD). In the case offabricating a semiconductor laser, a waveguide structure has beenincorporated into the epitaxially grown layer 3 by using a regrowthtechnique, selective etching or the like.

Next, in the step shown in FIG. 1B, the epitaxially grown layer 3 isadhered to an Si substrate 4 (recipient substrate) having a principalsurface substantially coincident with the (001) plane. In the step shownin FIG. 1C, the back surface of the sapphire substrate 1 is irradiatedwith a KrF excimer laser beam (at a wavelength of 248 nm correspondingto an energy of 5 eV), whereby the epitaxially grown layer 3 and thesapphire substrate 1 are separated from each other. The irradiation isperformed such that the beam (luminous flux) of the laser scans theentire surface of the wafer. The entire wafer has been heated to about500° C. such that an in-film stress resulting from the different thermalexpansion coefficients of the sapphire substrate 1, the ZnO layer 2, andthe epitaxially grown layer 3 are reduced. The heating temperature ispreferably in the range of 400° C. to 750° C. in terms of performing thestress reducing function without inducing the degradation of theproperties of the individual layers on the substrate and the significantdeformation thereof.

In the case of fabricating the semiconductor laser, the epitaxiallygrown layer 3 and the Si substrate 4 are adhered to each other such thatthe <11-20> direction of the GaN layer and the <110> direction of the Sisubstrate are in parallel relation for easy cleavage.

Consequently, the sapphire substrate 1 and the epitaxially grown layer 3are separated from each other, as shown in FIG. 1D. Thereafter, theprocess of forming an electrode which comes in contact with theepitaxially grown layer 3 on the Si substrate 4, cleaving the substrate(in the case of fabricating the semiconductor laser), and the like isperformed.

FIG. 2 is a cross-sectional view showing an example of a multilayerstructure formed in the step shown in FIG. 1A. In this example, astructure of a semiconductor laser is shown and the epitaxially grownlayer 3 has an n-GaN layer 3 a with a thickness of about 3 μm, ann-Al_(0.05)Ga_(0.95)N layer 3 b with a thickness of about 100 nm, adouble-well light-emitting region 3 c composed of threeIn_(0.05)Ga_(0.95)N layers each having a thickness of 6 nm and threeIn_(0.1)Ga_(0.9)N layers each having a thickness of about 3 nm which arealternately stacked, a p-Al_(0.05)Ga_(0.95)N layer 3 d with a thicknessof about 300 nm, and a p-GaN layer 3 e with a thickness of about 400 nm.

FIG. 3 shows an example of a multilayer structure formed in the stepshown in FIG. 1D. As shown in FIG. 3, the epitaxially grown layer 3shown in FIG. 2 is mounted in vertically inverted relation on the Sisubstrate 4. An electrode 10 for n-type conductivity composed of, e.g.,Ti/Al is further formed on the epitaxially grown layer 3.

It is to be noted that the epitaxially grown layer 3 according to thepresent invention is not limited to the structures shown in FIGS. 2 and3. The present invention is also applicable to a light-emitting diodehaving another structure, a semiconductor laser having anotherstructure, or a semiconductor device other than a semiconductor laserand having another structure such as a MESFET (Metal Semiconductor FieldEffect Transistor), a HEMT, or a Schottky diode.

FIG. 4 is an energy band diagram showing respective band structures inthe sapphire substrate 1, the ZnO layer 2 serving as the spacer layer,and the GaN layer included in the epitaxially grown layer 3 according tothe present embodiment. As shown in the drawing, the band gap (opticalband gap) of the ZnO layer 2 is 3.27 eV, which is smaller than the bandgap (3.39 eV) of the GaN layer.

In the present embodiment, the laser beam used to irradiate the backsurface of the wafer is primarily absorbed by the ZnO layer 2 so thatonly a small portion thereof reaches the epitaxially grown layer 3.Consequently, the decomposition or fusion of the crystal occurs in theentire ZnO layer 2 or in the region of the ZnO layer 2 adjacent theinterface with the sapphire substrate 1. This allows the epitaxiallygrown layer 3 and the sapphire substrate 1 to be separated from eachother at a low light power density. Since the epitaxially grown layer 3is hardly fused, the occurrence of a crystal defect or a crack in theepitaxially grown layer 3 during the laser irradiation can besuppressed. Even if the thickness of the epitaxially grown layer 3 isreduced to 5 μm or less, therefore, the epitaxially grown layer 3 andthe sapphire substrate 1 can be separated from each other, while theepitaxially grown layer 3 retains its excellent crystalline property.Since the epitaxially grown layer 3 is as thin as about 5 μm, it ispossible to reduce the bowing of the substrate resulting from thedifferent thermal expansion coefficients of the epitaxially grown layer3 and the sapphire substrate 1 during the cooling of the substrate afterthe epitaxial growth. This allows easy, uniform, and highly reproducibleadhesion to the flat Si substrate 4 performed by using, e.g., metal.

The low light power density indicates a light power having a valuesmaller than a threshold power density of about 200 mJ/cm² at which,when the third harmonic of, e.g., a YAG laser is used, a GaN layer indirect contact with a sapphire substrate as in a conventional nitridesemiconductor device (see FIG. 16C) is separated therefrom.

The time at which the Si substrate 4 is adhered to the epitaxially grownlayer 3 may be prior to the irradiation with the laser beam, as in thepresent embodiment, or after the separation of the sapphire substrate 1with the irradiation with the laser beam.

Embodiment 2

FIGS. 5A to 5D are cross-sectional views illustrating a method forfabricating a semiconductor device according to a second embodiment ofthe present invention.

First, in the step shown in FIG. 5A, a sapphire substrate 1 (wafer)having a principal surface substantially coincident with the (0001)plane (c plane) is prepared. Then, an In_(0.1)Ga_(0.9)N layer 5 having athickness of about 30 nm and serving as a spacer layer is formed by,e.g., MOCVD on the sapphire substrate 1. Subsequently, an epitaxiallygrown layer 3 (with a thickness of 5 μm) having a multilayer structureincluding a GaN layer, an AlGaN layer, and an InGaN layer, each of whichis of p-type, of n-type, or undoped, is formed on the In_(0.1)Ga_(0.9)Nlayer 5 by MOCVD performed in the same apparatus. In the case offabricating a semiconductor laser, a waveguide structure has beenincorporated into the epitaxial layer 3 by using a regrowth technique,selective etching or the like.

Next, in the step shown in FIG. 5B, the epitaxially grown layer 3 isadhered to an Si substrate 4 (recipient substrate) having a principalsurface substantially coincident with the (001) plane. In the step shownin FIG. 5C, the back surface of the sapphire substrate 1 is irradiatedwith a KrF excimer laser beam (at a wavelength of 248 nm correspondingto an energy of 5 eV), whereby the epitaxially grown layer 3 and thesapphire substrate 1 are separated from each other. The irradiation isperformed such that the beam (luminous flux) of the laser scans theentire surface of the wafer. The entire wafer has been heated to about500° C. such that an in-film stress resulting from the different thermalexpansion coefficients of the sapphire substrate 1, theIn_(0.1)Ga_(0.9)N layer 5, and the epitaxially grown layer 3 arereduced. The heating temperature is preferably in the range of 400° C.to 750° C. in terms of performing the stress reducing function withoutinducing the degradation of the properties of the individual layers onthe substrate and the significant deformation thereof.

In the case of fabricating the semiconductor laser, the epitaxiallygrown layer 3 and the Si substrate 4 are adhered to each other such thatthe <11-20> direction of the GaN layer and the <110> direction of the Sisubstrate are in parallel relation for easy cleavage.

The time at which the Si substrate 4 is adhered to the epitaxially grownlayer 3 may be prior to the irradiation with the laser beam, as in thepresent embodiment, or after the separation of the sapphire substrate 1with the irradiation with the laser beam.

Consequently, the sapphire substrate 1 and the epitaxially grown layer 3are separated from each other, as shown in FIG. 5D. Thereafter, theprocess of forming an electrode which comes in contact with theepitaxially grown layer 3 on the Si substrate 4, cleaving the substrate(in the case of fabricating the semiconductor laser), and the like isperformed.

In the present embodiment, a structure provided with theIn_(0.1)Ga_(0.9)N layer 5 in place of the ZnO layer 2 of the structureshown in, e.g., each of FIGS. 2 and 3 is obtained in each of the stepsshown in FIGS. 5A and 5D.

FIG. 6 is an energy band diagram showing respective band structures inthe sapphire substrate 1, the In_(0.1)Ga_(0.9)N layer 5, and the GaNlayer included in the epitaxially grown layer 3 according to the presentembodiment. As shown in the drawing, the band gap (optical band gap) ofthe In_(0.1)Ga_(0.9)N layer 5 is 3.0 eV, which is smaller than the bandgap (3.39 eV) of the GaN layer.

Thus, the structure according to the present embodiment is obtained byreplacing the ZnO layer 2 (spacer layer) according to the firstembodiment with the In_(0.1)Ga_(0.9)N layer 5 and the laser beam used toirradiate the back surface of the wafer is primarily absorbed by theIn_(0.1)Ga_(0.9)N layer 5 so that only a small portion thereof reachesthe epitaxially grown layer 3. Consequently, the decomposition or fusionof the crystal occurs in the entire In_(0.1)Ga_(0.9)N layer 5 or in theregion of the In_(0.1)Ga_(0.9)N layer 5 adjacent the interface with thesapphire substrate 1. This allows the epitaxially grown layer 3 and thesapphire substrate 1 to be separated from each other at a low lightpower density. Since the epitaxially grown layer 3 is hardly fused, theoccurrence of a crystal defect or a crack in the epitaxially grown layer3 during the laser irradiation can be suppressed. Even if the thicknessof the epitaxially grown layer 3 is reduced to 5 μm or less, therefore,the epitaxially grown layer 3 and the sapphire substrate 1 can beseparated from each other, while the epitaxially grown layer 3 retainsits excellent crystalline property Since the thickness of theepitaxially grown layer 3 is as thin as about 5 μm, it is possible toreduce the bowing of the substrate resulting from the different thermalexpansion coefficients of the epitaxially grown layer 3 and the sapphiresubstrate 1 during the cooling of the substrate after epitaxial growth.This allows easy, uniform, and highly reproducible adhesion to the flatSi substrate performed by using, e.g., metal. That is, the same effectsas achieved by the first embodiment are achievable.

Since the band gap of the In_(0.1)Ga_(0.9)N layer 5 is narrower than theband gap of the ZnO layer 2 according to the present embodiment, thepresent embodiment performs separation at a temperature or a laser powerdensity lower than in the first embodiment and more effectivelysuppresses the occurrence of a crystal defect, the bowing of thesubstrate, and the like.

Embodiment 3

FIGS. 7A to 7D are cross-sectional views illustrating a method forfabricating a semiconductor device according to a third embodiment ofthe present invention.

First, in the step shown in FIG. 7A, a sapphire substrate 1 (wafer)having a principal surface substantially coincident with the (0001)plane (c plane) is prepared. Then, an In_(0.1)Ga_(0.9)N layer 5 having athickness of about 30 nm and serving as a spacer layer is formed by,e.g., MOCVD on the sapphire substrate 1. Subsequently, an epitaxiallygrown layer 3 (with a thickness of 5 μm) having a multilayer structureincluding a GaN layer, an AlGaN layer, and an InGaN layer, each of whichis of p-type, of n-type, or undoped, is formed on the In_(0.1)Ga_(0.9)Nlayer 5 by MOCVD performed in the same apparatus. In the presentembodiment, an Al_(0.1)Ga_(0.9)N layer is formed in direct contact withthe In_(0.1)Ga_(0.9)N layer 5. In the case of fabricating asemiconductor laser, a waveguide structure has been incorporated intothe epitaxial layer 3 by using a regrowth technique or the like.

Next, in the step shown in FIG. 7B, the epitaxially grown layer 3 isadhered to an Si substrate 4 (recipient substrate) having a principalsurface substantially coincident with the (001) plane. In the step shownin FIG. 7C, to the back surface of the sapphire substrate 1 isirradiated with the third harmonic (at a wavelength of 355 nmcorresponding to an energy of 3.49 eV) of a YAG laser or a bright linespectrum (at a wavelength of 365 nm corresponding to an energy of 3.39eV) from a low-pressure mercury vapor lamp, whereby the epitaxiallygrown layer 3 and the sapphire substrate 1 are separated from eachother. The irradiation is performed such that the beam (luminous flux)of the laser or the bright line spectrum from the low-pressure mercuryvapor lamp scans the entire surface of the wafer. The entire wafer hasbeen heated to about 500° C. such that an in-film stress resulting fromthe different thermal expansion coefficients of the sapphire substrate1, the In_(0.1)Ga_(0.9)N layer 5, and the epitaxially grown layer 3 arereduced. The heating temperature is preferably in the range of 400° C.to 750° C. in terms of performing the stress reducing function withoutinducing the degradation of the properties of the individual layers onthe substrate and the significant deformation thereof.

In the case of fabricating the semiconductor laser, the epitaxiallygrown layer 3 and the Si substrate 4 are adhered to each other such thatthe <11-20> direction of the GaN layer and the <110> direction of the Sisubstrate are in parallel relation for easy cleavage.

The time at which the Si substrate 4 is adhered to the epitaxially grownlayer 3 may be prior to the irradiation with the laser beam, as in thepresent embodiment, or after the separation of the sapphire substrate 1with the irradiation with the laser beam.

Consequently, the sapphire substrate 1 and the epitaxially grown layer 3are separated from each other, as shown in FIG. 7D. Thereafter, theprocess of forming an electrode which comes in contact with theepitaxially grown layer 3 on the Si substrate 4, cleaving the substrate(in the case of fabricating the semiconductor laser), and the like isperformed.

In the present embodiment, a structure provided with theIn_(0.1)Ga_(0.9)N layer 5 in place of the ZnO layer 2 of the structureshown in, e.g., each of FIGS. 2 and 3 is obtained in the steps shown inFIGS. 7A and 7D.

FIG. 8 is an energy band diagram showing respective band structures inthe sapphire substrate 1, the In_(0.1)Ga_(0.9)N layer 5, and the GaNlayer in the epitaxially grown layer 3 according to the presentembodiment. As shown in the drawing, the band gap E1 (optical band gap)of the In_(0.1)Ga_(0.9)N layer 5 is 3.0 eV, which is smaller than theband gap (3.57 eV) of the Al_(0.1)Ga_(0.9)N layer, while the band gap E2of the Al_(0.1)Ga_(0.9)N layer is smaller than the band gap E0 of thesapphire substrate 1. In the present embodiment, the energy h ν of thelaser beam used for the separation is higher than the band gap E1 of theIn_(0.1)Ga_(0.9)N layer 5 and lower than the band gap E2 of theAl_(0.1)Ga_(0.9)N layer.

Thus, the structure according to the present embodiment is obtained byreplacing the ZnO layer 2 according to the first embodiment with theIn_(0.1)Ga_(0.9)N layer 5 and the laser beam used to irradiate the backsurface of the wafer is primarily absorbed by the In_(0.1)Ga_(0.9)Nlayer 5 so that only a small portion thereof reaches the epitaxiallygrown layer 3. Consequently, the decomposition or fusion of the crystaloccurs in the entire In_(0.1)Ga_(0.9)N layer 5 or in the region of theIn_(0.1)Ga_(0.9)N layer 5 adjacent the interface with the sapphiresubstrate 1. This allows the epitaxially grown layer 3 and the sapphiresubstrate 1 to be separated from each other at a low light powerdensity. Since the epitaxially grown layer 3 is hardly fused, theoccurrence of a crystal defect or a crack in the epitaxially grown layer3 can be suppressed. Even if the thickness of the epitaxially grownlayer 3 is reduced to 5 μm or less, therefore, the epitaxially grownlayer 3 and the sapphire substrate 1 can be separated from each other,while the epitaxially grown layer 3 retains its excellent crystallineproperty.

Since the thickness of the epitaxially grown layer 3 is as thin as about5 μm, it is possible to reduce the bowing of the substrate resultingfrom the different thermal expansion coefficients of the epitaxiallygrown layer 3 and the sapphire substrate 1 during the cooling of thesubstrate after epitaxial growth. This allows easy, uniform, and highlyreproducible adhesion to the flat Si substrate 4 performed by using,e.g., metal. That is, the same effects as achieved by the first andsecond embodiments are achievable.

Since the band gap difference (0.57 eV) between the In_(0.1)Ga_(0.9)Nlayer 5 and the Al_(0.1)Ga_(0.9)N layer is larger than the band gapdifference (0.12 eV) between the ZnO layer 2 and the GaN layer in thefirst embodiment and the band gap difference (0.39 eV) between theIn_(0.1)Ga_(0.9)N layer 5 and the GaN layer in the second embodiment,the present embodiment more effectively suppresses the occurrence of acrystal defect, the bowing of the substrate, and the like than the firstand second embodiments.

If the third harmonic (at a wavelength of 355 nm) of the YAG laser isused, in particular, the laser beam is absorbed by the In_(0.1)Ga_(0.9)Nlayer 5 but is hardly absorbed by the Al_(0.1)Ga_(0.9)N layer formingthe lowermost portion of the epitaxially grown layer so that only theregion of the In_(0.1)Ga_(0.9)N layer 5 adjacent the interface with thesapphire substrate 1 is decomposed or fused more effectively.

Variation

FIG. 18 is an energy band diagram showing respective band structures ina sapphire substrate, an epitaxially grown layer, and the like accordingto a variation of the third embodiment.

In the present variation, a GaN layer 5′ with a thickness of 0.3 μm isused in place of the In_(0.1)Ga_(0.9)N layer 5. The basic structure ofthe epitaxially grown layer 3 is a multilayer structure including a GaNlayer, an AlGaN layer, and an InGaN layer, each of which is of p-type,of n-type, or undoped, and having a thickness of 1 μm in the same manneras in the third embodiment. In the present variation, anAl_(0.1)Ga_(0.9)N layer is formed in direct contact with the GaN′ layer5′.

In the present variation, the band gap E1′ (optical band gap) of the GaNlayer 5′ is 3.4 eV, which is smaller than the band gap E2 (3.57 eV) ofthe Al_(0.1)Ga_(0.9)N layer, while the band gap E2 of theAl_(0.1)Ga_(0.9)N layer is smaller than the band gap E0 of the sapphiresubstrate 1. In the present variation also, the energy h ν of the laserbeam used for the separation is larger than the band gap E1′ of the GaNlayer 5′ and smaller than the band gap E2 of the Al_(0.1)Ga_(0.9)Nlayer. Accordingly, the present variation can achieve the same effectsas achieved by the third embodiment.

In addition to the effects achieved by the third embodiment, the presentvariation can also achieve the following effects. As shown in FIG. 17,the conventional semiconductor device having the single GaN layerprovided on the sapphire substrate 101 had a problem that a lower limitto the total thickness of the entire epitaxially grown layer 103 was 4μm by any means and, if the thickness of the epitaxially grown layer 103was further reduced, a crack or peeling-off is developed. This may bebecause air generated in the lower end portion of the epitaxially grownlayer or a stress released therein gives an impact.

By contrast, the present variation can reduce the total thickness (whichis 1.3 μm in the present variation even if the GaN′ layer 5′ is regardedas a part of the epitaxially grown layer) of the epitaxially grown layer3 to a value smaller than 4 μm. Consequently, the bowing of the entirewafer resulting from the different thermal expansion coefficients of thesapphire substrate and the epitaxially grown layer (nitridesemiconductor layer) can be suppressed so that a flat recipientsubstrate is adhered to the wafer. If a light-emitting diode, asemiconductor layer, or a high-temperature and high-speed transistor isformed by using such a thinned epitaxially grown layer, the reducedthickness of the epitaxially grown layer allows a device with animproved property, such as a reduced DC series resistance, to beobtained.

Embodiment 4

FIGS. 9A to 9D are cross-sectional views illustrating a method forfabricating a semiconductor device according to a fourth embodiment ofthe present invention.

First, in the step shown in FIG. 9A, a sapphire substrate 1 (wafer)having a principal surface substantially coincident with the (0001)plane (c plane) is prepared. Then, an AlN buffer layer 6 with athickness of about 30 nm is formed on the sapphire substrate 1 by MOCVDat a low temperature of about 500° C. and an In_(0.1)Ga_(0.9)N layer 5having a thickness of about 30 nm and serving as a spacer layer isfurther formed on the AlN buffer layer 6 by MOCVD. Subsequently, anepitaxially grown layer 3 (with a thickness of 5 μm) having a multilayerstructure including a GaN layer, an AlGaN layer, and an InGaN layer,each of which is of p-type, of n-type, or undoped, is formed on theIn_(0.1)Ga_(0.9)N layer 5 by MOCVD performed in the same depositionsystem. In the present embodiment, the GaN layer included in theepitaxially grown layer 3 is formed in direct contact with theIn_(0.1)Ga_(0.9)N layer 5. In the case of fabricating a semiconductorlaser, a waveguide structure has been incorporated into the epitaxiallayer 3 by using a regrowth technique, selective etching or the like.

Next, in the step shown in FIG. 9B, the epitaxially grown layer 3 isadhered to an Si substrate 4 (recipient substrate) having a principalsurface substantially coincident with the (001) plane. In the step shownin FIG. 9C, the back surface of the sapphire substrate 1 is irradiatedwith a KrF excimer laser beam (at a wavelength of 248 nm correspondingto an energy of 5 eV), whereby the epitaxially grown layer 3 and thesapphire substrate 1 are separated from each other. The irradiation isperformed such that the beam (luminous flux) of the laser scans theentire surface of the wafer. The entire wafer has been heated to about500° C. such that an in-film stress resulting from the different thermalexpansion coefficients of the sapphire substrate 1, theIn_(0.1)Ga_(0.9)N layer 5, and the epitaxially grown layer 3 arereduced. The heating temperature is preferably in the range of 400° C.to 750° C. in terms of performing the stress reducing function withoutinducing the degradation of the properties of the individual layers onthe substrate and the significant deformation thereof.

In the case of fabricating the semiconductor laser, the epitaxiallygrown layer 3 and the Si substrate 4 are adhered to each other such thatthe <11-20> direction of the GaN layer and the <110> direction of the Sisubstrate are in parallel relation for easy cleavage.

The time at which the Si substrate 4 is adhered to the epitaxially grownlayer 3 may be prior to the irradiation with the laser beam, as in thepresent embodiment, or after the separation of the sapphire substrate 1with the irradiation with the laser beam.

Consequently, the sapphire substrate 1 and the epitaxially grown layer 3are separated from each other, as shown in FIG. 9D. Thereafter, theprocess of forming an electrode which comes in contact with theepitaxially grown layer 3 on the Si substrate 4, cleaving the substrate(in the case of fabricating the semiconductor laser), and the like isperformed.

In the present embodiment, a structure in which the AlN buffer layer 6is provided on the sapphire substrate 1 and the In_(0.1)Ga_(0.9)N layer5 is provided in place of the ZnO layer 2 of the structure shown in,e.g., each of FIGS. 2 and 3 is obtained in the steps shown in FIGS. 9Aand 9D.

FIG. 10 is an energy band diagram showing respective band structures inthe sapphire substrate 1, the AlN buffer layer 6, the In_(0.1)Ga_(0.9)Nlayer 5, and the GaN layer included in the epitaxially grown layer 3according to the present embodiment. As shown in the drawing, the bandgap E1 (optical band gap) of the In_(0.1)Ga_(0.9)N layer 5 is 3.0 eV,which is smaller than the band gap E4 (3.39 eV) of the GaN layer and theband gap E3 (6.1 eV) of the AlN buffer layer 6. The energy h ν of thelaser beam used for peeling is larger than the band gap E1 of theIn_(0.1)Ga_(0.9)N layer 5 and smaller than the band gap E3 (6.1 eV) ofthe AlN buffer layer 6.

Thus, the structure according to the present embodiment is obtained byreplacing the ZnO layer 2 according to the first embodiment with theIn_(0.1)Ga_(0.9)N layer 5 and the laser beam used to irradiate the backsurface of the wafer is primarily absorbed by the In_(0.1)Ga_(0.9)Nlayer 5 so that only a small portion thereof reaches the epitaxiallygrown layer 3. Consequently, the decomposition or fusion of the crystaloccurs in the entire In_(0.1)Ga_(0.9)N layer 5 or in the region of theIn_(0.1)Ga_(0.9)N layer 5 adjacent the interface with the sapphiresubstrate 1. This allows the epitaxially grown layer 3 and the sapphiresubstrate 1 to be separated from each other at a low light powerdensity. Since the epitaxially grown layer 3 is hardly fused, theoccurrence of a crystal defect or crack in the epitaxially grown layer 3can be suppressed. Even if the thickness of the epitaxially grown layer3 is reduced to 5 μm or less, therefore, the epitaxially grown layer 3and the sapphire substrate 1 can be separated from each other, while theepitaxially grown layer 3 retains its excellent crystalline property.Since the thickness of the epitaxially grown layer 3 is as thin as about5 μm, it is possible to reduce the bowing of the substrate resultingfrom the different thermal expansion coefficients of the epitaxiallygrown layer 3 and the sapphire substrate 1 during the cooling of thesubstrate after epitaxial growth. This allows easy, uniform, and highlyreproducible adhesion to the flat Si substrate 4 performed by using,e.g., metal. That is, the same effects as achieved by the first, second,and third embodiments are achievable.

Since the band gap difference (0.39 eV) between the In_(0.1)Ga_(0.9)Nlayer 5 and the GaN layer is larger than the band gap difference (0.12eV) between the ZnO layer 2 and the GaN layer in the first embodiment,the present embodiment performs separation at a temperature lower thanin the first embodiment and more effectively suppresses the occurrenceof a crystal defect, the bowing of the substrate, and the like than thefirst embodiment.

Since the present embodiment is provided with the AlN buffer layer 6which is a buffer layer for reducing distortion resulting from a latticemismatch between the sapphire substrate 1 and the In_(0.1)Ga_(0.9)Nlayer 5 as the spacer layer, the distortion resulting from the latticemismatch therebetween is reduced and the crystalline properties of theIn_(0.1)Ga_(0.9)N layer 5 and the epitaxially grown layer 3 are improvedcompared with the case where the In_(0.1)Ga_(0.9)N layer 5 is formeddirectly on the sapphire substrate 1. Compared with the first to thirdembodiments, therefore, the present embodiment further improves theproperty (which is, e.g., a luminous intensity or an operating currentin a semiconductor laser) of a device formed by using the epitaxiallygrown layer 3.

Variation

FIG. 19 is an energy band diagram showing respective band structures ina sapphire substrate, an epitaxially grown layer, and the like accordingto a variation of the fourth embodiment.

In the variation, an AlN buffer layer 6′ with a thickness of 1 μm isformed in place of the AlN buffer layer 6 with a thickness of about 30nm. On the other hand, a spacer layer 5″ composed of anIn_(0.1)Ga_(0.9)N layer or a GaN layer having a thickness of 0.3 μm isformed on the AlN buffer layer 6′ in place of the In_(0.1)Ga_(0.9)Nlayer 5 with a thickness of about 30 nm. The basic structure of theepitaxially grown layer 3 is a multilayer structure including a GaNlayer, an AlGaN layer, and an InGaN layer, each of which is of p-type,of n-type, or undoped, and having a thickness of 1 μm in the same manneras in the fourth embodiment. The thickness of the epitaxially grownlayer 3 is 2 μm. In the present variation, an Al_(0.4)Ga_(0.6)N layer isformed in direct contact with the spacer layer 5″.

In the present variation, the band gap E1″ of the spacer layer 5″ is 3.4eV or 3.0 eV, which is smaller than the band gap E4′ (4.3 eV) of theAl_(0.4)Ga_(0.6)N layer and the band gap E3′ (6.1 eV) of the AlN bufferlayer 6′. The energy h ν of the laser beam used for peeling is largerthan the band gap E1″ of the spacer layer 5″ and smaller than the bandgap E3′ of the AlN buffer layer 6′. Accordingly, the present variationcan achieve the same effects as achieved by the fourth embodiment.

In addition to the effects achieved by the fourth embodiment, thepresent variation also achieves the following effects. That is, thepresent variation can reduce the total thickness (which is 2.3 μm in thepresent variation even if the spacer layer 5″ is regarded as a part ofthe epitaxially grown layer) of the epitaxially grown layer 3 to a valuesmaller than 4 μm, similarly to the variation of the third embodiment.Consequently, the bowing of the entire wafer resulting from thedifferent thermal expansion coefficients of the sapphire substrate andthe epitaxially grown layer (nitride semiconductor layer) can besuppressed so that a flat recipient substrate is adhered to the wafer.By thus increasing the thickness of the AlN buffer layer 6′ fromconventional 50 nm to 1 μm, the crystalline property of the epitaxiallygrown layer can be improved. In contrast to the dislocation density ofthe epitaxially grown layer (nitride semiconductor layer) which is about10⁹/cm² in the structure according to the fourth embodiment, thedislocation density of the epitaxially grown layer (nitridesemiconductor layer) in the present variation is about 10⁸/cm², so thata dislocation density lower by one order of magnitude is achieved. Topositively achieve such a crystalline-property improving effect, thethickness of the AlN buffer layer 6′ is preferably 1 μm or more.

Embodiment 5

FIGS. 11A to 11D are cross-sectional views illustrating a method forfabricating a semiconductor device according to a fifth embodiment ofthe present invention.

First, in the step shown in FIG. 11A, a sapphire substrate 1 (wafer)having a principal surface substantially coincident with the (0001)plane (c plane) is prepared. Then, a ZnO layer 2 with a thickness ofabout 100 nm serving as a spacer layer is formed on the sapphiresubstrate 1 by, e.g., RF sputtering. Thereafter, an AlGaN multilayerportion 7 composed of five Al_(0.05)Ga_(0.95)N layers each having athickness of about 30 nm and five Al_(0.1)Ga_(0.9)N layers each having athickness of about 30 nm, which are alternately stacked, is formed byMOCVD on the ZnO layer 2. Subsequently, an epitaxially grown layer 3(with a thickness of 5 μm) having a multilayer structure including a GaNlayer, an AlGaN layer, and an InGaN layer, each of which is of p-type,of n-type, or undoped, is formed by MOCVD on the AlGaN multilayerportion 7. In the present embodiment, the GaN layer included in theepitaxially grown layer 3 is formed in direct contact with the AlGaNmultilayer portion 7. In the case of fabricating a semiconductor laser,a waveguide structure has been incorporated into the epitaxial layer 3by using a regrowth technique, selective etching or the like.

Next, in the step shown in FIG. 11B, the epitaxially grown layer 3 isadhered to an Si substrate 4 (recipient substrate) having a principalsurface substantially coincident with the (001) plane. In the step shownin FIG. 11C, the back surface of the sapphire substrate 1 is irradiatedwith the third-harmonic (at a wavelength of 355 nm corresponding to anenergy of 3.49 eV) of a YAG laser, whereby the sapphire substrate 1 andthe epitaxially grown layer 3 are separated from each other. Theirradiation is performed such that the beam (luminous flux) of the laserscans the entire surface of the wafer. The entire wafer has been heatedto about 500° C. such that an in-film stress resulting from thedifferent thermal expansion coefficients of the sapphire substrate 1,the ZnO layer 2, the multilayer portion 7, and the epitaxially grownlayer 3 are reduced. The heating temperature is preferably in the rangeof 400° C. to 750° C. in terms of performing the stress reducingfunction without inducing the degradation of the properties of theindividual layers on the substrate and the significant deformationthereof.

In the case of fabricating the semiconductor laser, the epitaxiallygrown layer 3 and the Si substrate 4 are adhered to each other such thatthe <11-20> direction of the GaN layer and the <110> direction of the Sisubstrate are in parallel relation for easy cleavage.

The optical density (at a wavelength of 355 nm) of the third harmonic ofthe YAG laser is preferably 200 mJ/cm² or more.

The time at which the Si substrate 4 is adhered to the epitaxially grownlayer 3 may be prior to the irradiation with the laser beam, as in thepresent embodiment, or after the separation of the sapphire substrate 1with the irradiation with the laser beam.

Consequently, the sapphire substrate 1 and the epitaxially grown layer 3are separated from each other, as shown in FIG. 11D. Thereafter, theprocess of forming an electrode which comes in contact with theepitaxially grown layer 3 on the Si substrate 4, cleaving the substrate(in the case of fabricating the semiconductor laser), and the like isperformed.

In the present embodiment, a structure provided with the AlGaNmultilayer portion 7 in place of the ZnO layer 2 of the structure shownin, e.g., each of FIGS. 2 and 3 is obtained in the steps shown in FIGS.11A and 11D.

FIG. 12 is an energy band diagram showing respective band structures inthe sapphire substrate 1, the ZnO layer 2, the AlGaN multilayer portion7, and the GaN layer included in the epitaxially grown layer 3 accordingto the present embodiment.

In the present embodiment also, the laser beam used to irradiate theback surface of the wafer is primarily absorbed by the ZnO layer 2 sothat it hardly reaches the AlGaN multilayer portion 7 and theepitaxially grown layer 3. Consequently, the decomposition or fusion ofthe crystal occurs in the entire ZnO layer 2 or in the region of the ZnOlayer 2 adjacent the interface with the sapphire substrate 1. Thisallows the epitaxially grown layer 3 and the sapphire substrate 1 to beseparated from each other at a low light power density. Since theepitaxially grown layer 3 is hardly fused, the occurrence of a crystaldefect or crack in the epitaxially grown layer 3 can be suppressed.Since the thickness of the epitaxially grown layer 3 is as thin as about5 μm, it is possible to reduce the bowing of the substrate resultingfrom the different thermal expansion coefficients of the epitaxiallygrown layer 3 and the sapphire substrate 1 during the cooling of thesubstrate after epitaxial growth. This allows easy, uniform, and highlyreproducible adhesion to a flat Si substrate 4 performed by using, e.g.,metal.

In addition, a crystal defect or a crack caused in the ZnO layer 2 bythe laser beam used for irradiation is terminated at the interfacesbetween the individual layers of the AlGaN multilayer portion 7 composedof the five Al_(0.05)Ga_(0.95)N layers and the five Al_(0.1)Ga_(0.9)Nlayers which are alternately stacked, as shown in the enlarged view ofFIG. 11D. This more effectively prevents the extension of the crystaldefect or crack to the epitaxially grown layer 3 and improves thecrystalline property of the epitaxially grown layer 3. Compared with thefirst to third embodiments, therefore, the present embodiment canfurther improve the property (which is, e.g., a luminous intensity or anoperating current in a semiconductor laser) of a device formed by usingthe epitaxially grown layer 3.

The multilayer portion 7 need not necessarily be composed of two thinfilms having different compositions as in the present embodiment. Themultilayer portion 7 may be formed by alternately stacking three or morethin films in layers or may be composed of a plurality of thin filmshaving different compositions. This is because, if the thin films formedone after another have gradually varying compositions, the extension ofa crystal defect or a crack is prevented at the boundaries between theformed thin films.

In the case of providing the multilayer portion 7, the epitaxially grownlayer need not necessarily be separated from the substrate with theirradiation with the laser beam. It is also possible to use anepitaxially grown layer grown epitaxially on the substrate as it is. Inthat case, the spacer layer such as the ZnO layer 2 need not necessarilybe provided. Since crystal defects or cracks in the epitaxially grownlayer is reduced by the formation of the epitaxially grown layer on themultilayer portion 7 compared with those in the conventional device, thepresent embodiment achieves the effect of improving the property (whichis, e.g., a luminous intensity or an operating current in asemiconductor laser) of a device formed by using the epitaxially grownlayer 3.

Variation

The GaN multilayer portion 7 may also be formed to have a multiplequantum well structure. FIG. 13 is a cross-sectional view showing avariation of the present embodiment in which the AlGaN multilayerportion 7 is formed to have a multiple quantum well structure. As shownin FIG. 13, the epitaxially grown layer 3 has been separated from thesapphire substrate and mounted on an Si substrate 4. The multilayerportion 7, the ZnO layer 2, and an n-type electrode 10 are providedsuccessively on the epitaxially grown layer 3. The structure is obtainedafter the epitaxially grown layer 3 and the multilayer portion 7 wereseparated from the sapphire substrate and bowinged onto the Sisubstrate. However, a method for separating the epitaxially grown layer3 and the multilayer portion 7 from the sapphire substrate is notlimited to the method used in the present embodiment. The ZnO layer 2serving as the spacer layer need not necessarily be provided. In asemiconductor laser, the epitaxially grown layer 3 has a structure asshown in FIG. 3.

In this variation, the AlGaN multilayer portion 7 has a structureobtained by alternately stacking five Al_(0.05)Ga_(0.95)N layers 7 aeach heavily doped with a p-type impurity and having a thickness ofabout 2 nm and five undoped Al_(0.1)Ga_(0.9)N layers 7 b each having athickness of about 10 nm. By thus adopting a structure in which each ofthe plurality of heavily doped quantum wells is interposed between theundoped layers, a low-resistance p-type region can be formed, whiledefective regions are reduced. Even if a large number of crystal defectsand cracks are present in a bulk single-crystal substrate, therefore,the extension of the crystal defects and cracks in the single-crystalsubstrate to the semiconductor layer is prevented in the multilayerportion so that the semiconductor layer with an excellent crystallineproperty is obtainable. Since the efficiency with which carries aresupplied to the epitaxially grown layer 3 is increased, the property ofa device (which is, e.g., a luminous intensity or an operating currentin a semiconductor laser) can be increased, while the DC seriesresistance is reduced.

Embodiment 6

FIGS. 14A to 14D are cross-sectional views illustrating a method forfabricating a semiconductor device according to a sixth embodiment ofthe present invention.

First, in the step shown in FIG. 14A, a sapphire substrate 1 (wafer)having a principal surface substantially coincident with the (0001)plane (c plane) is prepared. Then, an epitaxially grown layer 3 having amultilayer structure including a GaN layer, an AlGaN layer, and an InGaNlayer, each of which is of p-type, of n-type, or undoped, is formed onthe ZnO layer 2. Subsequently, covering portions 8 each composed of atungsten layer or an SiO₂/TiO₂ multilayer film having a thickness ofabout 100 nm are formed within the epitaxially grown layer 3.Specifically, after a GaN layer with a thickness of, e.g., about 1 μm isformed on the ZnO layer 2, the tungsten film or SiO₂/TiO₂ multilayerfilm is formed by, e.g., RF sputtering on the GaN layer. The tungstenfilm or SiO₂/TiO₂ multilayer film is patterned by, e.g., reactive ionetching to form the striped covering portions 8 arranged in stripes eachhaving a width of about 5 μm with 10-μm spaces provided therebetween.Instead of the striped covering portions 8, dotted (island-like)covering portions may also be provided. Subsequently, the GaN layer, theAlGaN layer, and the InGaN layer are grown epitaxially by, e.g., MOCVDfrom the portions of the GaN layer located in the spaces between thestripes of the covering portions 8, whereby the epitaxially grown layer3 having a multilayer structure with the total thickness of about 6 μmis formed. In the present embodiment, the GaN layer included in theepitaxially grown layer 3 is formed in direct contact with the ZnO layer2. In the case of forming a semiconductor laser, a waveguide structurehas been incorporated into the epitaxial layer 3 by using a regrowthtechnique, selective etching or the like.

It is also possible to form the covering portions 8 directly on the ZnOlayer 2 serving as the spacer layer and then growing the epitaxiallygrown layer 3 from the portions of the ZnO layer located in the spacesbetween the stripes of the covering portions 8.

Next, in the step shown in FIG. 14B, the epitaxially grown layer 3 isadhered to an Si substrate 4 (recipient substrate) having a principalsurface substantially coincident with the (001) plane. In the step shownin FIG. 14C, the back surface of the sapphire substrate 1 is irradiatedwith a KrF excimer laser (at a wavelength of 248 nm), whereby thesapphire substrate 1 and the epitaxially grown layer 3 are separatedfrom each other. The irradiation is performed such that the beam(luminous flux) of the laser scans the entire surface of the wafer. Theentire wafer has been heated to about 500° C. such that an in-filmstress resulting from the different thermal expansion coefficients ofthe sapphire substrate 1, the ZnO layer 2, the multilayer portion 7, andthe epitaxially grown layer 3 are reduced. The heating temperature ispreferably in the range of 400° C. to 750° C. in terms of performing thestress reducing function without inducing the degradation of theproperties of the individual layers on the substrate and the significantdeformation thereof.

In the case of fabricating the semiconductor laser, the epitaxiallygrown layer 3 and the Si substrate 4 are adhered to each other such thatthe <11-20> direction of the GaN layer and the <110> direction of the Sisubstrate are in parallel relation for easy cleavage.

The time at which the Si substrate 4 is adhered to the epitaxially grownlayer 3 may be prior to the irradiation with the laser beam, as in thepresent embodiment, or after the separation of the sapphire substrate 1with the irradiation with the laser beam.

The optical density of the KrF excimer laser is preferably 600 mJ/cm² ormore.

Consequently, the sapphire substrate 1 and the epitaxially grown layer 3are separated from each other, as shown in FIG. 14D. Thereafter, theprocess of forming an electrode which comes in contact with theepitaxially grown layer 3 on the Si substrate 4, cleaving the substrate(in the case of fabricating the semiconductor laser), and the like isperformed.

FIG. 15 is a cross-sectional view showing an exemplary structure of anSiO₂/TiO₂ multilayer film 8 x. As shown in FIG. 15, the SiO₂/TiO₂multilayer film 8 x is composed of four SiO₂ films 8 a having athickness of 59.7 nm and four TiO₂ films 8 b having a thickness of 59.7nm which are alternately stacked in layers. The structure achieves ahigh reflectivity of 99.5% against a UV light beam at 355 nm (e.g., thethird harmonic of a YAG laser).

In the present embodiment also, the laser beam used to irradiate theback surface of the wafer is primarily absorbed by the ZnO layer 2 sothat it hardly reaches the AlGaN multilayer portion 7 and theepitaxially grown layer 3. Consequently, the decomposition or fusion ofthe crystal occurs in the entire ZnO layer 2 or in the region of the ZnOlayer 2 adjacent the interface with the sapphire substrate 1. Thisallows the epitaxially grown layer 3 and the sapphire substrate 1 to beseparated from each other at a low light power density. Since thethickness of the epitaxially grown layer 3 is as thin as about 5 μm, itis possible to reduce the bowing of the substrate resulting from thedifferent thermal expansion coefficients of the epitaxially grown layer3 and the sapphire substrate 1 during the cooling of the substrate afterepitaxial growth. This allows easy, uniform, and highly reproducibleadhesion to a flat Si substrate 4 performed by using, e.g., metal.

In addition, the fabrication method according to the present embodimenthas formed the covering portions 8 composed of the tungsten film orSiO₂/TiO₂ multilayer film within the epitaxially grown layer 3corresponding to the nitride semiconductor layer according to the firstembodiment. If a tungsten layer is inserted as the covering portions 8,the laser beam is reflected by the tungsten layer and confined in theportions of the GaN layer and ZnO layer located under the tungstenlayer, which allows the separation of the sapphire substrate 1 and theepitaxially grown layer 3 at a lower power density. If the SiO₂/TiO₂multilayer film is used as the covering portions 8, the reflectivityagainst the laser beam can also be increased by adjusting the filmthicknesses of the SiO₂ film and the TiO₂ film so that the same effectsas achieved by the tungsten film are achievable.

If the SiO₂/TiO₂ multilayer film is used as the covering portions 8, inparticular, the thermal conductivity thereof is lower than that of theGaN layer so that the function of confining heat raised by the laserbeam in the separating step is enhanced. This allows the separation ofthe sapphire substrate 1 and the epitaxially grown layer 3 at a lowerpower density.

Moreover, the present embodiment has formed the GaN layer, the AlGaNlayer, and the InGaN layer from the GaN layer as the underlie after theformation of the covering portions 8. In that case, if the epitaxiallygrown crystal layer grows through the spaces between the stripes of thecovering portions 8, the epitaxially grown layer 3 is formed by upwardcrystal growth as well as lateral crystal growth along the uppersurfaces of the covering portions 8. In such a structure, even if acrystal defect and a crack occur in the GaN layer in contact with theZnO layer 2 after crystal growth and extend to the crystal layer growingupwardly of the GaN layer, it has been proved that they seldom extend tothe laterally growing crystal layer. As a consequence, the portions ofthe epitaxially grown layer 3 overlying the covering portions 8 arehardly affected by the underlie so that the crystal defect and crackextended from the underlie are seldom observed therein. This indicatesthat the fabrication method according to the present embodiment allowsthe epitaxial growth of a crystal with a particularly excellentcrystalline property.

Thus, the fabrication method according to the present embodiment allowsthe separation of the sapphire substrate 1 and the epitaxially grownlayer 3 at a light power density of the laser beam lower than in thefirst embodiment and further reduces crystal defects and cracks in theepitaxially grown layer 3.

Embodiment 7

FIG. 20 is an energy band diagram showing respective band structures ina sapphire substrate, an epitaxially grown layer, and the like accordingto a seventh embodiment of the present invention.

In the present embodiment, an AlN buffer layer 6′ with a thickness of 1μm is formed instead of the AlN buffer layer 6 with a thickness of about30 nm according to the fourth embodiment. Instead of the spacer layer 5″composed of the In_(0.1)Ga_(0.9)N layer or the GaN layer provided in thefourth embodiment, an epitaxially grown layer 3 including a thick GaNlayer is formed on the AlN buffer layer 6′. The basic structure of theepitaxially grown layer 3 has a multilayer structure including a GaNlayer, an AlGaN layer, and an InGaN layer, each of which is of p-type,of n-type, or undoped in the same manner as in the fourth embodiment.The thickness of the epitaxially grown layer 3 is 4 μm. In the presentembodiment, the GaN layer is formed in direct contact with the AlNbuffer layer 6′.

In the present embodiment, the band gap E5 of the GaN layer included inthe epitaxially grown layer 3 is 3.4 eV, which is smaller than the bandgap E3′ (6.1 eV) of the AlN buffer layer 6′. The energy h ν of the laserbeam used for peeling is larger than the band gap E5 (3.4 eV) of the GaNlayer and smaller than the band gap E3′ (6.1 eV) of the AlN buffer layer6′.

By thus increasing the thickness of the AlN buffer layer 6′ fromconventional 50 nm to 1 μm, the present embodiment can improve thecrystalline property of the epitaxially grown layer, similarly to thevariation of the fourth embodiment. In contrast to the dislocationdensity of the epitaxially grown layer (nitride semiconductor layer)which is about 10⁹/cm² in the structure according to the fourthembodiment, the dislocation density of the epitaxially grown layer(nitride semiconductor layer) in the present variation is about 10⁸/cm²,so that a dislocation density lower by one order of magnitude isachieved. To positively achieve such a crystalline-property improvingeffect, the thickness of the AlN buffer layer 6′ is preferably 1 μm ormore.

Other Embodiments

The structure formed in accordance with the method for fabricating asemiconductor device and the process thereof according to each of theforegoing embodiments and the variations thereof achieves particularlyprominent effects if it is applied to a semiconductor device having agroup III-V compound semiconductor layer. The types of the semiconductordevice to which the structure according to the present invention isapplicable include a light-emitting diode, a semiconductor laser, and asemiconductor device other than the semiconductor laser such as aMESFET, HEMT, or a Schottky diode.

In the present invention, a substrate (single-crystal substrate) servingas the underlie for each of the epitaxially grown layers is not limitedto the sapphire substrate. It is also possible to use an SiC substrate,an MgO substrate, an LiGaO₂ substrate, an LiGa_(x)Al_(1-x)O₂ (0≦x≦1)mixed crystal substrate, or the like. The use of the sapphire substrateimproves the initial growth of a group III-V compound and allows a groupIII-V compound semiconductor layer containing nitrogen and having anexcellent crystalline property to be formed thereon. The use of the SiCsubstrate, the MgO substrate, the LiGaO₂ substrate, or theLiGa_(x)Al_(1-x)O₂ (0≦x≦1) mixed crystal substrate brings the latticeconstant of the single-crystal substrate closer to that of the groupIII-V compound semiconductor layer and allows the semiconductor layercomposed of a group III-V compound, containing nitrogen, and having anexcellent crystalline property to be formed thereon.

Although the present invention has bowinged the epitaxially grown layerseparated from the sapphire substrate onto the Si substrate (recipientsubstrate), it is also possible to use a GaAs substrate, a GaPsubstrate, an InP substrate, or the like as the recipient substrate inplace of the Si substrate. This is because an excellent cleavage planecan easily be obtained by using such a single-crystal substrate.

Although each of the foregoing embodiments has transferred theepitaxially grown layer separated from the sapphire substrate onto theSi substrate, it is also possible to use the separated epitaxially grownlayer as it is without mounting it on another substrate.

1. A method for fabricating a semiconductor device having asemiconductor layer formed by epitaxial growth from a single-crystalsubstrate, the method comprising the steps of: (a) forming a spacerlayer having an optical band gap smaller than an optical band gap of alowermost portion of the semiconductor layer such that an upper surfaceof the single-crystal substrate is covered with the spacer layer; (b)forming the semiconductor layer on the spacer layer; and (c) irradiatingthe spacer layer with a light beam having an energy smaller than anoptical band gap of the single-crystal substrate and larger than theoptical band gap of the spacer layer through a back surface of thesingle-crystal substrate to separate the semiconductor layer from thesingle-crystal substrate, wherein the step (b) includes forming a groupIII-V compound semiconductor layer as the semiconductor layer, whereinthe step (a) includes forming a ZnO layer as the spacer layer and thestep (b) includes composing the lowermost portion of the semiconductorlayer of a group III-V compound material containing nitrogen and havingan optical band gap larger than an optical band gap of the ZnO layer. 2.A method for fabricating a semiconductor device having a semiconductorlayer formed by epitaxial growth from a single-crystal substrate, themethod comprising the steps of: (a) forming a spacer layer having anoptical band gap smaller than an optical band gap of a lowermost portionof the semiconductor layer such that an upper surface of thesingle-crystal substrate is covered with the spacer layer; (b) formingthe semiconductor layer on the spacer layer; and (c) irradiating thespacer layer with a light beam having an energy smaller than an opticalband gap of the single-crystal substrate and larger than the opticalband gap of the spacer layer through a back surface of thesingle-crystal substrate to separate the semiconductor layer from thesingle-crystal substrate, wherein the step (a) includes forming, as thespacer layer, a group III-V compound semiconductor layer containingnitrogen and the step (b) includes composing the lowermost portion ofthe semiconductor layer of a group III-V compound semiconductor layercontaining nitrogen and having an optical band gap larger than theoptical band gap of the spacer layer, wherein the step (a) includesforming a GaN layer as the spacer layer and the step (b) includescomposing the lowermost portion of the semiconductor layer of anAl_(y)Ga_(1-y)N layer (0<y≦1), and wherein the step (b) includesadjusting a thickness of the semiconductor layer to a range not lessthan 0.5 μm and less than 4 μm.
 3. A method for fabricating asemiconductor device having a semiconductor layer formed by epitaxialgrowth from a single-crystal substrate, the method comprising the stepsof: (a) forming a spacer layer having an optical band gap smaller thanan optical band gap of a lowermost portion of the semiconductor layersuch that an upper surface of the single-crystal substrate is coveredwith the spacer layer; (b) forming the semiconductor layer on the spacerlayer; and (c) irradiating the spacer layer with a light beam having anenergy smaller than an optical band gap of the single-crystal substrateand larger than the optical band gap of the spacer layer through a backsurface of the single-crystal substrate to separate the semiconductorlayer from the single-crystal substrate, said method further comprising,after the step (a) and prior to the step (b), the step of: forming, onthe spacer layer, a multilayer portion composed of a plurality of thinfilms stacked in layers to have gradually varying compositions, whereinthe step (b) includes forming the semiconductor layer on the multilayerportion.
 4. The method of claim 3, wherein the multilayer portion is amultiple quantum well layer composed of alternately stacked quantum welllayers and barrier layers.
 5. A method for fabricating a semiconductordevice having a semiconductor layer formed by epitaxial growth from asingle-crystal substrate, the method comprising the steps of: (a)forming a spacer layer having an optical band gap smaller than anoptical band gap of a lowermost portion of the semiconductor layer suchthat an upper surface of the single-crystal substrate is covered withthe spacer layer; (b) forming the semiconductor layer on the spacerlayer; and (c) irradiating the spacer layer with a light beam having anenergy smaller than an optical band gap of the single-crystal substrateand larger than the optical band gap of the spacer layer through a backsurface of the single-crystal substrate to separate the semiconductorlayer from the single-crystal substrate, wherein the step (b) includesthe substeps of: (b1) forming a plurality of covering portions coveringthe spacer layer in mutually spaced apart relation; and (b2) forming thesemiconductor layer such that the spacer layer and the plurality ofcovering portions are covered with the semiconductor layer.
 6. Themethod of claim 5, wherein the substep (b2) includes the steps of: priorto the substep (b1), forming, in a part of the semiconductor layercovering the spacer layer, the plurality of covering portions inmutually spaced apart relation; and after the substep (b1), forming aremaining part of the semiconductor layer through a space between thecovering portions.
 7. The method of claim 5, wherein the substep (b1)includes forming the covering portions composed of a multilayerinsulating film or a metal film.
 8. The method of claim 5, wherein thesubstep (b1) includes forming the covering portions composed of amaterial lower in thermal conductivity than the spacer layer.
 9. Amethod for fabricating a semiconductor device having a semiconductorlayer formed by epitaxial growth from a single-crystal substrate, themethod comprising the steps of: (a) forming a spacer layer having anoptical band gap smaller than an optical band gap of a lowermost portionof the semiconductor layer such that an upper surface of thesingle-crystal substrate is covered with the spacer layer; (b) formingthe semiconductor layer on the spacer layer; and (c) irradiating thespacer layer with a light beam having an energy smaller than an opticalband gap of the single-crystal substrate and larger than the opticalband gap of the spacer layer through a back surface of thesingle-crystal substrate to separate the semiconductor layer from thesingle-crystal substrate, said method further comprising, prior to thestep (a), the step of: forming, on the single-crystal substrate, abuffer layer having an optical band gap larger than the energy of thelight beam used for the irradiation in the step (c) such that the bufferlayer reduces distortion resulting from a lattice mismatch between thespacer layer and the single-crystal substrate in the step (c), whereinthe step (a) includes forming the spacer layer on the buffer layer. 10.The method of claim 9, wherein the step of forming the buffer layerincludes forming, as the buffer layer, an AlN buffer layer having athickness in the range of 0.5 μm to 2 μm.
 11. A method for fabricating asemiconductor device having a semiconductor layer formed by epitaxialgrowth from a single-crystal substrate, the method comprising the stepsof: (a) forming a spacer layer having an optical band gap smaller thanan optical band gap of a lowermost portion of the semiconductor layersuch that an upper surface of the single-crystal substrate is coveredwith the spacer layer; (b) forming the semiconductor layer on the spacerlayer; and (c) irradiating the spacer layer with a light beam having anenergy smaller than an optical band gap of the single-crystal substrateand larger than the optical band gap of the spacer layer through a backsurface of the single-crystal substrate to separate the semiconductorlayer from the single-crystal substrate, said method further comprising,prior to the step (a), the step of: forming, on the single-crystalsubstrate, an AlN buffer layer having a thickness of 0.5 μm or more,wherein the step (a) includes forming an In_(x)Ga_(1-x)N layer (0<x≦1)or a GaN layer as the spacer layer and the step (b) includes forming thesemiconductor layer on the spacer layer such that the lowermost portionof the semiconductor layer is composed of an Al_(y)Ga_(1-y)N layer(0<y≦1).
 12. A method for fabricating a semiconductor device, the methodcomprising the steps of: (a) forming an AlN buffer layer having athickness of 0.5 μm or more on a single-crystal substrate; (b) forming asemiconductor layer covering the AlN buffer layer and having a lowermostportion composed of an Al_(y)Ga_(1-y)N layer (0≦y≦1); and (c)irradiating the semiconductor layer with a light beam having an energysmaller than an optical band gap of the AlN buffer layer and larger thanan optical band gap of the lowermost portion of the semiconductor layerthrough a back surface of the single-crystal substrate to separate thesemiconductor layer from the single-crystal substrate.